The present application is based on Japanese priority application No.2000-291144 filed on Sep. 25, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to magnetic storage of information and more particularly to a magnetic storage medium for use in high-density magnetic information storage devices.
With recent advancement in the field of information processing, magnetic disk devices, particularly those used in computers and other information processing apparatuses as external or auxiliary information storage device, are exposed to a stringent demand of more recording density, more resolution, and higher signal-to-noise ratio.
In a general magnetic recording medium for a longitudinal magnetic recording, a pulse width Pw50 of a reproduced magnetic signal is defined as
Pw50=(2(a+d)2+(a/2)2)xc2xd
axe2x88x9d(txc3x97Br/Hc)xc2xdxe2x80x83xe2x80x83(1) 
wherein Hc represents a coercive force of a magnetic layer provided in the magnetic recording medium, Br represents a remnant magnetic flux density in the magnetic layer, t represents the thickness of the magnetic layer, and d represents a magnetic spacing between the magnetic layer and a magnetic head.
The narrower the width Pw50 of the magnetic pulse, the better the resolution of the reproduced signal. Thus, in order to increase the recording density and resolution of the magnetic storage medium, it is effective to reduce the thickness t of the magnetic layer and increase the coercive force Hc thereof.
Meanwhile, there is another demand for a high-density magnetic storage medium, in relation to the requirement of minimizing a medium noise, in that the magnetic layer has a high S/Nm (signal-to-medium noise) ratio. In order to suppress the medium noise, it has been practiced to reduce the grain size of the magnetic particles in the magnetic layer and suppress the magnetic interaction between the magnetic particles as much as possible.
For example, Japanese Laid-Open Patent Publication 3-31638 describes a magnetic storage medium that uses a magnetic layer of a Co-alloy film containing therein Cr and Ta with respective concentration levels of 6-20 at % (atomic percent) and 9 at %, wherein improvement is achieved in the foregoing prior art magnetic storage medium with regard to the S/Nm ratio by incorporating Cu with a concentration level of 0.5-7 at %. By doing so, it is possible to reduce the particle size in the Co alloy film used in the magnetic storage medium as the magnetic layer.
However, the demand for higher density recording has become more stringent these days, and it was discovered by the inventor of the present invention that the magnetic layer of the foregoing prior art composition can no longer meet for the demand of recent, leading-edge magnetic storage devices. Further, no solution has been proposed conventionally for improving anisotropy magnetic field Hk and for preventing degradation of the coercive force Hc under the situation that the product (txc3x97Br) is set small for improved resolution and for improved S/Nm ratio.
In a magnetic storage media for use in high-density magnetic storage devices, it is noted that there is a serious problem known as thermal fluctuation. When the thickness of the magnetic layer is reduced or the grain size of the magnetic crystals therein is reduced extremely for improved resolution and improved S/Nm ratio, there is a tendency that magnetic relaxation is promoted in the magnetic layer and the remnant magnetization of the magnetic layer is degraded as a result.
Thus, the phenomenon of thermal fluctuation has to be suppressed as much as possible particularly in the case of high-density magnetic storage medium, while this minimization of the thermal fluctuation has to be accompanied with simultaneous minimization of the product (txc3x97Br) for minimization of medium noise and also for simultaneous improvement of resolution.
It is known that the relaxation time xcfx84 of a magnetic layer is represented, according to the Nee-Arrhenius relationship as
xcfx84xe2x88x921=f0 exp(xe2x88x92xcex94E/kT) 
xcex94E=Kuxc2x7Vxc2x7(1xe2x88x92H/H0)1/n; n=⅔, 
Ku=Hkxc2x7Ms/2, H=He+Hd,xe2x80x83xe2x80x83(2) 
where f0 represents a spin precession frequency having an order of 109/s, k represents Boltzmann""s constant, T represents a temperature of the magnetic layer, Ku represents an anisotropy energy constant, V represents an effective volume of a magnetic particle in the magnetic layer, H0 represents an intrinsic coercive force in the absence of thermal fluctuation, Ms represents a saturated magnetization, He represents an external magnetic field, Hd represents a demagnetization field at the bit transition, and Hk represents an anisotropy magnetic field.
Referring to Eq.(2) above, it is noted that the attempt to reduce the medium noise by reducing the product (txc3x97Br) invites a decrease of the relaxation time xcfx84 by way of causing a reduction of the particle volume V or causing reduction of the saturation magnetic field Ms. When the relaxation time xcfx84 is reduced, the resistance of the magnetic layer against thermal fluctuation is degraded and the strength of the output signal reproduced from the magnetic storage medium may become smaller with time.
In view of the foregoing, a demand has emerged recently for a technology that can maintain a high value for the product (Kuxc3x97V), so that a sufficient resistance is maintained against thermal fluctuation while simultaneously minimizing the product (txc3x97Br).
It should be noted that the coercive force Hc of a magnetic layer is a function of temperature and time. Thus, the coercive force Hc appears low when the measurement of the coercive force is conducted at a high temperature.
A remnance coercive force Hcr is given as
Hcr/H0=1xe2x88x92{Cxc2x7ln(f0xc2x7tim/ln2)}n 
Cxe2x88x921=xcex94E/kTxe2x80x83xe2x80x83(3) 
where tim represents the duration in which the external magnetic field He is applied.
From Eq.(3), it can be seen that the coercive force Hc is degraded in the magnetic recording medium susceptible to thermal fluctuation when the product (txc3x97Br) is reduced. With the degradation of the coercive force Hc, the resistance to thermal fluctuation is degraded and also the S/Nm ratio.
As noted previously, it is desirable to design the magnetic layer of the magnetic storage medium such that a large coercive force Hc is maintained even when the product (txc3x97Br) is set small. However, various magnetic properties of the magnetic layer are interrelated, and thus, it has been extremely difficult to make a magnetic storage medium that maintains a high coercive force Hc for the magnetic layer therein even when the value of the product (txc3x97Br) is reduced.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic storage medium wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a magnetic storage medium for high-density magnetic recording having a large coercive force Hc for a magnetic layer therein even in such a case the value of the product (txc3x97Br) is set small.
Another object of the present invention is to provide a magnetic storage medium, comprising:
a non-magnetic substrate;
an under layer provided on said non-magnetic substrate; and
at least one magnetic layer provided above said under layer,
said magnetic layer comprising at least an alloy layer of a system Coxe2x80x94Crxe2x80x94Ptxe2x80x94Bxe2x80x94Cu,
said alloy layer having a thickness t and a remnant magnetic flux density Br satisfying a relationship for a product (txc3x97Br) as
2.0 nTxc2x7mxe2x89xa6(txc3x97Br)xe2x89xa67.0 nTxc2x7m, 
said alloy layer containing, in addition to Co, Cr with a concentration xcex2 of 20-26 at % (20 at %xe2x89xa6xcex2xe2x89xa626 at %), Pt with a concentration xcex3 of 6-20 at % (6 at %xe2x89xa6xcex3xe2x89xa620 at %), B with a concentration xcex4 of 1-7 at % (1 at %xe2x89xa6xcex4xe2x89xa67 at %), and Cu with a concentration xcex5 of 2-7 at % (2 at %xe2x89xa6xcex5xe2x89xa67 at %).
According to the present invention, it is possible to maintain a desired coercive force Hc of about 2000 (xc3x97xc2xcxcfx80xc2x7kA/m) or more even in such a case the product (txc3x97Br) of the magnetic layer is reduced to the level of 2.0-7.0 nTxc2x7m (nano-Teslaxc2x7meter). Thus, the present invention can achieve the improvement in the resolution at the time of reading and the elimination of medium noise simultaneously. The magnetic recording medium of the present invention further has a superior magnetic anisotropy and is resistant against thermal fluctuation. In the case the total of the atomic percentages of Cr, Pt, B and Cu in the magnetic layer is 55 at % (xcex2+xcex3+xcex4+xcex5=55 at %), the magnetic layer contains Co with a concentration of 45 at %.
It should be noted that the magnetic layer may have a thickness t of 10-25 nm in the magnetic storage medium of the present invention, provided that the product (txc3x97Br) falls in the foregoing range between 2.0 and 7.0 nTxc2x7m. The magnetic layer may be included in the magnetic storage medium in one or more layers. By providing the magnetic layer in two or more layers, it is possible to reduce the remnant magnetic flux density Br further while maintaining the high coercive force Hc. Alternatively, the magnetic storage medium may contain the magnetic layer as one of a plurality of magnetic layers therein.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.